First off: welcome back to STO. Next week is a new feature episode as part of the 3rd Anniversary celebrations and that should be fun.
Secondly, I think Section 31 would be a good fit for you based on your post. It has a number of features:
1) fun and casual - you won't be forced to attend meetings, donate X to projects, or log Y times a week (see note below*). This isn't a job, we just want you to have fun.
2) large - over 495 members in fleet
3) active - 100% of the members have logged in within 60 days (* if you have a habit of going away for months at a time, this is unfortunately not a fleet for you) and there's almost always 10-40 members online at any time of day
4) advanced fleet holdings - one of the top fleets in the game, Section 31 has all Tier 4 upgrades complete for the starbase and will be completely maxed on the embassy in February. We also don't make you wait weeks or months to access the fleet store.
5) misc - we have a KDF sister fleet (310+ members, level 13 starbase) and a public chat channel to talk to people if you're on a different faction (KDF), server (Tribble), Cryptic game (Champions Online), or not logged in (Pidgin)
Additional info at the link in my signature. I can be reached in-game @PhyrexianHero